Portable apparatus, such as, for example, patient lifting devices are well known in the medical field for transferring patients between varying locations, such as, for example, between a bed and a gurney or between a test station and a wheelchair. These devices commonly include a base section having an attached hydraulic or other form of lifting mechanism. This lifting mechanism typically includes an boom arm having an articulating end that is attached by means of a spreader arm, bar or other load supporting member to a body sling, wheelchair, or similar lifting patient support. The spreader arm attempts to evenly distribute the weight of the patient support and includes receiving means on opposing ends to which straps or other connecting means from the patient support are attached. Stationary types of the above devices, such as ceiling mounted versions, are also commonly known in the field.
There are several manufacturers of various apparatuses, such as the above noted patient lifting devices and patient transfer devices that now each use an electronic scale as an accessory to their lifts. These electronic scales are discrete assemblies that separately incorporate a tensile load cell or similar axial force transducer whose output can be converted for readout onto a display. Though some success has been achieved in having an electronic scale with a patient lifting device, there are a number of disadvantages in using “off the shelf” electronic scales in conjunction therewith.
A first noted disadvantage in incorporating so-called “off the shelf” electronic scale assemblies is that the overall lifting height of the lifting device is decreased because the scale accessory is typically attached between the lift boom and the spreader bar or other load supporting member. This attachment decreases the overall effectiveness of the patient lifting device and also increases the overall lifting height and the angle of the lift boom which may also adversely affect the center of gravity of the device.
A second disadvantage created in using an attached electronic scale is that inaccuracies are induced into the scale because the scale is restricted from movement in at least one or more directions. When electronic scales having a tensile load cell or similar axial force transducer design are not permitted to hang freely from the lift boom, a side load or torque is created, thereby skewing the pure tensile load that is created by the patient. As a result, indirect loads are transmitted to the electronic scale assembly and inaccurate readings are displayed.
Yet a third disadvantage is that there are other inaccuracies that can be induced into the electronic scale when the scale is not oriented vertically. That is to say, if the line of force through a load cell is not vertical, an error is produced that is proportional to the horizontal force component. The latter problem is also produced when attempting to incorporate an electronic scale into a spreader bar or other load supporting member.
There are additional problems or disadvantages which arise when attempting to incorporate an electronic scale into a spreader bar. For example, if the spreader bar travel is restricted, side loads will be introduced. In addition and if the spreader bar permits the patient's center of gravity to travel outside of the support structure of the patient lifting device, the device could become unstable wherein possible serious injury could result.